This invention relates generally to combustible cartridges and more particularly to an improved case construction permitting the use of threaded case components.
Combustible cases for large caliber ammunition have been used for a number of years. The advantage of using such cases is especially apparent in tank ammunition where disposal of spent cartridge cases is constrained by the cramped confines within the fighting vehicle. The combustible cases used in such ammunition are designed to be consumed during propellant ignition, leaving behind only the short metal cartridge head containing the spent primer. Storage and handling spent cartridge heads measuring about three inches in length and five inches in diameter is much more convenient than wrestling with two foot hot long metal cases.
Combustible cartridge cases have a tubular body made of a molded or wrapped cardboard type mixture of wood fiber, Kraft cellulose, nitrocellulose, and a stabilizer. The cases are formed by the conventional beater additive or post impregnation process. The outer surface of the case body is impregnated with a plastic resin such as polyurethane to protect the case from humidity and abrasion damage and add strength to the case. The case has a density gradient of resin across the wall thickness of the case. The resin content is minimum at the inside surface and maximum at the outside. This gradient is conventionally believed to be necessary in order to ensure complete case combustion since the resin is a combustion inhibitor.
The tubular body is conventionally glued to a combustible adapter which attaches to the obturator of a projectile having its rear end extending through the adapter into the combustible cartridge case body. The adapter is a generally tubular cone which necks the case down to the bore diameter of the gun. The rear end of the adapter is glued to the case body and the front end of the adapter is bolted, glued or snap fit onto the obturator of the projectile.
The adapter is made of the same mixture as the case but usually without the nitrocellulose. The conventional adapter also has the same plastic resin gradient across its wall thickness.
The projectile is usually a shape charge warhead or a subcaliber kinetic energy long rod penetrator for defeating armor. It has its rear end extending into the forward end cf the combustible case through the adapter. The opposite end of the combustible case has a generally cup shaped metal head mechanically attached to it which provides a gun breech seal and a support for the primer which extends into the propellant within the combustible case.
These rounds have historically been very susceptable to joint failures during normal handling, changes in extreme humidity/temperature conditions and most importantly, accidental dropping. Accordingly, many efforts have been made to find the best joint arrangement and to improve the strength of these joints.
Such attempts are exemplified in the following patents. U.S. Pat. No. 3,978,792 to Campoli et al discloses a cartridge case adapter which fits over a fin hub for a fin stabilized projectile. The adapter is designed for use in a polypropylene cartridge case. The adapter is made of a polycarbonate resin material and engages the fins and rigidly supports them against the interior of the case. Thus, in this design, the adapter fits down inside the case and radially supports the finned projectile from the rear. This prevents the balloting of the long rod penetrator projectile within the case minimizing the forces placed on the joint between the projectile and the combustible case.
In U.S. Pat. Nos. 4,187,783 and 4,444,113, the sabots are snap fit to the obturator of the sabot.
In U.S. Pat. No. 4,444,115, the projectile is connected to the combustible casing via two axially spaced supports. The body portion of the projectile is supported at the neck of the case. A second, rear radial support is provided by radial support elements within the case abutting the case wall at the rear of the projectile.
In U.S. Pat. No. 4,487,131, a cartridge cover or adapter is disclosed which has a plurality of axial slots in its inverted conical forward end. These slots receive corresponding catches on the rear of the sabot on the subcaliber projectile as the projectile is inserted into the case. Thus, the forward end of the adapter is snap fit into engagement with the sabot. In addition, a silicon rubber seal is disposed between the case adapter and the sabot to provide a form adapted joint between the cartridge and the projectile. The rear portion of the cartridge cover or adapter has a conventional skive joint for gluing to the forward open end of the combustible case. This arrangement has the inherent disadvantage that the connection between the sabot of the projectile and the adapter is not particularly rigid.
U.S. Pat. Nos. 3,981,246 and 4,714,024 disclose conventional bolting methods between the combustible case adapter in the rear portion of the sabot. These bolted connections are strong but are relatively complex and require substantial assembly time. In addition, connection failures between the adapter and the sabot may not be readily apparent from a visual inspection.
Attempts to utilize threaded connections for combustible case components over twenty years ago met with dismal failure. Because of the resin density and thus a structural strength gradient across the wall thickness of the case, and the limited thickness of the case wall, machined threads simply disintegrated. Accordingly, threaded connections were believed to be unsuitable for use in combustible case components.
The preferred conventional joint between the adapter and the case body is a skive joint in which the case and the adapter are glued together. The skive joint presents an optimum glue surface area. The joints between the projectile and the adapter are either a snap fit, a compression fit between sabot components, a glued joint or a bolted connection between the adapter and the skirt of the obturator or the sabot.
One such conventional prior art connection scheme between the projectile and the combustible case is illustrated in FIG. 2. Cartridge 1 comprises a projectile 2 mounted to a combustible case 3 via adapter 4. Adapter 4 has a skive joint glued to case 3 at its rear end and has a bolted connection 5 at its forward end 6 to the sabot 7 of the projectile 2. The forward end 6 of the adapter 4 is shaped as a reverse cone having an angle corresponding to the tapered rear of sabot 7. A plurality of bolts 8 extend through both the forward end 6 of the adapter 4 and the rear tapered portion of the sabot 7 to connect the two pieces together.
This type of connection between the projectile and the adapter is strong but particularly susceptible to hidden joint failures in the region of the bolted connections. In addition, this type of connection is difficult and time consuming to assemble because of the recessed position of the bolts.
Both of these joints, the case to adapter and the adapter to projectile have frequently failed drop tests or have failed during field use over the years. This has thus been a long term problem with this particular type of cartridge. Accordingly, a need has existed for a better and more reliable method to fasten the combustible case components together.
It is therefore an object of the invention to provide an improved combustible case material into which threads can be machined.
It is another object of the present invention to provide an improved connection between combustible case components.
It is another object of the present invention to provide an improved connection between the projectile and the combustible case.